In recent years it has become common practice, particularly in clinical oncology, to simultaneously administer several drugs to patients for therapeutic purposes. A vast literature, both experimental and clinical, has shown that the presence of one drug may markedly influence the intensity, duration, and pharmacologic effects of another drug. Such drug interactions may have a variety of causes: they may result from alterations in drug absorption, in the binding of drugs to plasma proteins, which may directly or indirectly influence drug distribution, drug metabolism, or drug clearance via the bile or urine. Many drugs and other foreign chemical compounds, known collectively as xenobiotics, are inactivated by enzymes present in liver microsomes, primarily by oxidation and conjugation. On the other hand, some drugs and oncolytic agents are activated by these enzymes. Our major efforts are directed at understanding how drugs interact with each other and with the biological organism so that human drug therapy can be placed on a more rational basis.